System of abnormality detection for oxygen concentration sensor

ABSTRACT

A system of abnormality detection for an oxygen concentration sensor employed to sense the oxygen concentration in engine exhaust gas, and to obtain data to control the air/fuel ratio of a fuel mixture supplied to the engine. The oxygen concentration sensor including a sensor cell element and an oxygen pump element. The system serves to detect an abnormality such as an open-circuit or a short-circuit in electrode connecting leads of the sensor cell element on the basis of a value of voltage developed between the electrodes of the sensor cell element and of a value of current which flows between the electrodes of the oxygen pump element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system of abnormality detection foran oxygen concentration sensor which senses the level of oxygenconcentration in a gas such as engine exhaust gas.

2. Description of Background Information

In order to reduce exhaust gas pollutants and to improve the fuelconsumption of an internal combustion engine, it is now common practiceto employ an oxygen concentration sensor to detect the concentration ofoxygen in the engine exhaust gas, and to execute feedback control of theair/fuel ratio of the mixture supplied to the engine such as to hold theair/fuel ratio at a target value. This feedback control is performed inaccordance with an output signal from the oxygen concentration sensor.

One form of oxygen concentration sensor which can be employed for suchair/fuel ratio control functions by producing an output signal whichvaries in level in accordance with the oxygen concentration in theengine exhaust gas. Such an oxygen concentration sensor has beendisclosed for example in Japanese patent laid-open No. 52-72286. Thissensor consists of an oxygen ion-conductive solid electrolytic memberformed as a flat plate having a pair of electrodes formed on two mainfaces, with one of these electrode faces forming part of a gas holdingchamber. The gas holding chamber communicates with a gas which is to bemeasured, i.e. exhaust gas, through a lead-in aperture. With such anoxygen concentration sensor, the oxygen ion-conductive solidelectrolytic member and its electrodes function as an oxygen pumpelement. By passing a flow of current between the electrodes such thatthe electrode within the gas holding chamber becomes a negativeelectrode, oxygen gas within the gas holding chamber adjacent to thisnegative electrode becomes ionized, and flows through the solidelectrolytic member towards the positive electrode, to be therebyemitted from that face of the sensor element as gaseous oxygen. Thecurrent flow between the electrodes is a boundary current value which issubstantially constant, i.e. is substantially unaffected by variationsin the applied voltage, and is proportional to the oxygen concentrationwithin the gas under measurement. Thus, by sensing the level of thisboundary current, it is possible to measure the oxygen concentrationwithin the gas which is under measurement. However, if such an oxygenconcentration sensor is used to control the air/fuel ratio of themixture supplied to an internal combustion engine by measuring theoxygen concentration within the engine exhaust gas, then it will only bepossible to control the air/fuel ratio to a value which is in the leanregion relative to the stoichiometric air/fuel ratio. It is not possibleto perform air/fuel ratio control to maintain a target air/fuel ratiowhich is set in the rich region. An oxygen concentration sensor whichwill provide an output signal level varying in proportion to the oxygenconcentration in engine exhaust gas for both the lean region and therich region of the air/fuel ratio has been proposed in Japanese patentlaid-open No. 59-192955. This sensor consists of two oxygenion-conductive solid electrolytic members each formed as a flat plate,and each provided with a pair of electrodes. Two opposing electrodefaces, i.e. one face of each of the solid electrolytic members, formpart of a gas holding chamber which communicates with a gas undermeasurement, via a lead-in aperture. The other electrode of one of thesolid electrolytic members faces into the atmosphere. In this oxygenconcentration sensor, one of the solid electrolytic members and its pairof electrodes function as an oxygen concentration ratio sensor cellelement. The other solid electrolytic member and its pair of electrodesfunction as an oxygen pump element. If the voltage which is generatedbetween the electrodes of the oxygen concentration ratio sensor cellelement is higher than a reference voltage value, then current issupplied between the electrodes of the oxygen pump element such thatoxygen ions flow through the oxygen pump element towards the electrodeof that element which is within the gas holding chamber. If the voltagedeveloped between the electrodes of the sensor cell element is lowerthan the reference voltage value, then a current is supplied between theelectrodes of the oxygen pump element such that oxygen ions flow throughthat element towards the oxygen pump element electrode which is on theopposite side of the gas holding chamber. In this way, a value ofcurrent flow between the electrodes of the oxygen pump element isobtained which varies in proportion to the oxygen concentration of thegas under measurement, both in the rich and the lean regions of theair/fuel ratio.

However, with such an oxygen concentration sensor, if an abnormality ofthe sensor should occur, then not only will it be impossible to achievea desired oxygen concentration sensing characteristic, but it will be nolonger possible to accurately control the air/fuel ratio, so that theeffectiveness of exhaust gas pollution removal will be reduced. It istherefore desirable to provide a method of reliably detecting anyabnormality of components of such an oxygen concentration sensor.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a system ofdetecting abnormality of an oxygen concentration sensor, and inparticular to provide a method which is capable of reliably detectingany abnormality of a system of electrical connections of a sensor cellelement of an oxygen concentration sensor.

According to a first aspect, the present invention comprises a system ofabnormality detection for an oxygen concentration sensor which includestwo pairs of electrodes disposed mutually opposing with the pairs eachsandwiching an oxygen ion-conductive solid electrolytic member, gasdiffusion control means communicating with a gas under measurement forleading said gas to the vicinity of one electrode of each of said twopairs of electrodes, and control means for applying a pump voltage thatis determined in accordance with a voltage difference between a sensorvoltage which is produced between a first pair of said two pairs ofelectrodes and a reference voltage between a second pair of said twopairs of electrodes to maintain said sensor voltage at said referencevoltage. Thereby said control means produces an output which representsan oxygen concentration sensing value by a value of pump current whichflows between said second pair of electrodes. The system beingcharacterized in that an abnormality of an electrical connection systemof said first pair of electrode is sensed based upon a voltage developedbetween said first pair of electrodes and a current which flows betweensaid second pair of electrodes.

According to a second aspect, the present invention comprises a systemof abnormality detection for an oxygen concentration sensor of anair/fuel ratio control apparatus for an internal combustion engine. Saidsensor comprising two pairs of electrodes disposed mutually opposingwith the pairs each sandwiching an oxygen ion-conductive solidelectrolytic member, gas diffusion control means communicating with aexhaust gas of said internal combustion engine for leading said gas tothe vicinity of one electrode of each of said two pairs of electrodes,and control means for applying a pump voltage that is determined inaccordance with a voltage difference between a sensor voltage which isproduced between a first pair of said two pairs of electrodes and areference voltage between a second pair of said two pairs of electrodesto maintain said sensor voltage at said reference voltage. Thereby saidcontrol means produces an output which represents an oxygenconcentration sensing value by a value of pump current which flowsbetween said second pair of electrodes. Further, said control apparatuscomprises a means for obtaining an air/fuel ratio compensation valuebased upon said oxygen concentration sensing value for use incontrolling an air/fuel ratio of a mixture supplied to said internalcombustion engine to maintain said ratio at a target air/fuel ratio, anddrive means for driving air/fuel ratio adjustment means of said internalcombustion engine in accordance with a corrected air/fuel ratio controlvalue obtained by correcting said air/fuel ratio control value by saidair/fuel ratio compensation value. The system being characterized inthat an abnormality of an electrical connection system of said firstpair of electrode is sensed based upon a sensor voltage developedbetween said first pair of electrodes and upon said air/fuel ratiocompensation value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an electronic control fuel injectionapparatus equipped with an oxygen concentration sensor, suitable forapplication of the abnormality detection method of the presentinvention;

FIG. 2 is a diagram for illustrating the internal configuration of anoxygen concentration sensor detection unit;

FIG. 3 is a block circuit diagram of the interior of an ECU (ElectronicControl Unit), and;

FIGS. 4a, 4b, 4ba and 4bb are flow charts for assistance in describingthe operation of a CPU.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described, referringto the drawings. FIGS. 1 through 3 show an electronic fuel controlapparatus for an internal combustion engine incorporating an oxygenconcentration sensor which utilizes the output compensation system ofthe present invention. In this apparatus, an oxygen concentration sensordetection unit 1 is mounted within an exhaust pipe 3 of an engine 2,upstream from a catalytic converter 5. Inputs and outputs of the oxygenconcentration sensor detection unit 1 are coupled to an ECU (ElectronicControl Unit) 4.

The protective case 11 of the oxygen concentration sensor detection unitcontains an oxygen ion-conductive solid electrolytic member 12 which mayhave, for example, a substantially rectangular shape of the form shownin FIG. 2. A gas holding chamber 13 is formed in the interior of thesolid electrolytic member 12, and communicates via a lead-in aperture 14with exhaust gas at the exterior of solid electrolytic member 12,constituting a gas to be sampled. The lead-in aperture 14 is positionedsuch that the exhaust gas will readily flow from the interior of theexhaust pipe into the gas holding chamber 13. In addition, anatmospheric reference chamber 15 is formed within the solid electrolyticmember 12, into which atmospheric air is led. The atmospheric referencechamber 15 is separated from the gas holding chamber 13 by a portion ofthe solid electrolytic member 12 serving as a partition. As shown, pairsof electrodes 17a, 17b and 16a, 16b are respectively formed on thepartition between chambers 13 and 15 and on the wall of chamber 13opposite to chamber 15. The solid electrolytic member 12 functions inconjunction with the electrodes 16a and 16b as an oxygen pump element18, and functions in conjunction with electrodes 17a, 17b as a sensorcell element 19. A heater element 20 is mounted on the external surfaceof the atmospheric reference chamber 15.

The oxygen ion-conductive solid electrolytic member 12 can be formed,for example, of ZrO₂ (zirconium dioxide), while the electrodes 16athrough 17b are each formed of platinum.

As shown in FIG. 3, ECU 4 includes an oxygen concentration sensorcontrol section, consisting of a differential amplifier 21, a referencevoltage source 22, and resistors 23 and 24. Electrode 16b of the oxygenpump element 18 and electrode 17b of sensor cell element 19 are eachconnected to ground potential, and electrode 17a of sensor cell element19 is connected through resistor 24 to a supply voltage V_(cc).Electrode 17a is also connected to an input terminal of the differentialamplifier 21, which produces an output voltage in accordance with thedifference between a voltage developed across electrodes 17a and 17b ofthe sensor cell element 19 and the output voltage from the referencevoltage source 22. The output voltage from reference voltage source 22is a value corresponding to a stoichiometric air/fuel ratio (for example0.4 V). The output terminal of differential amplifier 21 is connectedthrough current sensing resistor 23 to electrode 16a of the oxygen pumpelement 18. The terminals of current sensing resistor 23 constitute apair of output terminals of the oxygen concentration sensor, and arecoupled to a microcomputer which constitutes the control circuit 25.

A throttle valve opening sensor 31 which produces an output voltage inaccordance with the degree of opening of throttle valve 26, which can beimplemented as a potentiometer, is coupled to control circuit 25 that isalso connected to an absolute pressure sensor 32 which is mounted inintake pipe 27 at a position downstream from the throttle valve 26 andproduces an output voltage varying in level in accordance with theabsolute pressure within the intake pipe 27. Control circuit 25 is alsoconnected to a water temperature sensor 33 which produces an outputvoltage varying in level in accordance with the temperature of theengine cooling water, and to a crank angle sensor 34 which produces asignal consisting of successive pulses respectively produced insynchronism with rotation of the crankshaft (not shown in the drawings)of engine 2. Control circuit 25 is also connected to an injector 35,provided in the intake pipe 27, near the intake valves (not shown in thedrawings) of engine 2.

Control circuit 25 includes an analog/digital (A/D) converter 39 forconverting the voltage V_(S) developed between electrodes 17a and 17b ofsensor cell element 19 into a digital signal, and an A/D converter 40receives the voltage developed across the current sensing resistor 23 asa differential input and converts that voltage to a digital signal.Control circuit 25 also includes a level converter circuit 41 whichperforms level conversion of each of the output signals from thethrottle valve opening sensor 31, the absolute pressure sensor 32, andthe water temperature sensor 33. The resultant level-converted signalsfrom level converter circuit 41 are supplied to inputs of a multiplexer42. Control circuit 25 also includes an A/D converter 43 which convertsthe output signals from multiplexer 42 to digital form, a waveformshaping circuit 44 which executes waveform shaping of the output signalfrom the crank angle sensor 34 to produce TDC (top dead center) signalpulses as output, and a counter 45 which counts a number of clock pulses(produced from a clock pulse generating circuit which is not shown inthe drawings) during each interval between successive TDC pulses fromthe waveform shaping circuit 44. Control circuit 25 further includes adrive circuit 46a for driving the injector 35, a pattern display drivecircuit 46b for driving a pattern display unit 38, a CPU (centralprocessing unit) 47 for performing digital computation in accordancewith a program with a ROM (read-only memory) 48 having variousprocessing programs and data stored therein, and a RAM (random accessmemory) 49. The A/D converters 39, 40 and 43, multiplexer 42, counter45, drive circuits 46a, 46b, CPU 47, ROM 48 and RAM 49 are mutuallyinterconnected by an input/output bus 50. The TDC signal is suppliedfrom the waveform shaping circuit 44 to the CPU 47. The control circuit25 also includes a heater current supply circuit 51 which, for example,can include a switching element that is responsive to a heater currentsupply command from CPU 47 for applying a voltage between the terminalsof heater element 20 to thereby supply heater current and produceheating of heater element 20.

The voltage V_(S) developed between electrodes 17a and 17b of the sensorcell element 19 that is transferred from A/D converter 39, datarepresenting a pump current value I_(P) corresponding to the currentflow through the oxygen pump element 18 that is transferred from A/Dconverter 40, and data representing a degree of throttle valve openingθ_(TH), the absolute pressure P_(BA) within the intake pipe, and thecooling water temperature T_(W) are respectively and selectivelytransferred by A/D converter 43 are supplied to CPU 47 over the I/O bus50. In addition, a count value from counter 45, that is attained duringeach period of the TDC pulses, is also supplied to CPU 47 over I/O bus50. The CPU 47 executes read-in of each of these data in accordance witha processing program which is stored in the ROM 48, and computes a fuelinjection time interval T_(OUT) for injector 35 on the basis of the datain accordance with a fuel injection quantity for engine 2 which isdetermined from predetermined equations. This computation is performedby means of a fuel supply routine, which is executed in synchronism withthe TDC signal. The injector 35 is then actuated by drive circuit 46afor the duration of the fuel injection time interval T_(OUT), to supplyfuel to the engine.

The fuel injection time interval T_(OUT) can be obtained for examplefrom the following equation:

    T.sub.OUT =T.sub.I ×K.sub.O2 ×K.sub.WOT ×K.sub.TW . . . (1)

In the above equation, T_(I) is the basic supply quantity, which isdetermined in accordance with the engine speed of rotation N_(e) and theabsolute pressure P_(BA) in the intake pipe and expresses a basicinjection time interval. K_(O2) is a feedback compensation coefficientfor the air/fuel ratio, which is set in accordance with the outputsignal level from the oxygen concentration sensor. K_(WOT) is a fuelquantity increment compensation coefficient, which is applied when theengine is operating under high load. K_(TW) is a cooling watertemperature coefficient. T_(I), K_(O2), K_(WO2) and K_(TW) arerespectively set by a subroutine of the fuel supply routine.

When the supply of pump current to the oxygen pump element begins, ifthe air/fuel ratio of the mixture which is supplied to engine 2 at thattime is in the lean region, then the voltage which is produced betweenelectrodes 17a and 17b of the sensor cell element 19 will be lower thanthe output voltage from the reference voltage source 22, and as a resultthe output voltage level from the differential amplifier 21 will bepositive. This positive voltage is applied through the series-connectedcombination of resistor 23 and oxygen pump element 18. A pump currentthereby flows from electrode 16a to electrode 16b of the oxygen pumpelement 18, so that the oxygen within the gas holding chamber 13 becomesionized by electrode 16b, and flows through the interior of oxygen pumpelement 18 from electrode 16b, to be ejected from electrode 16a asgaseous oxygen. Oxygen is thereby drawn out of the interior of the gasholding chamber 13.

As a result of this withdrawal of oxygen from the gas holding chamber13, a difference in oxygen concentration will arise between the exhaustgas within gas holding chamber 13 and the atmospheric air within theatmospheric reference chamber 15. A voltage V_(S) is thereby producedbetween electrodes 17a and 17b of the sensor cell element 19 at a leveldetermined by this difference in oxygen concentration, and the voltageV_(S) is applied to the inverting input terminal of differentialamplifier 21. The output voltage from differential amplifier 21 isproportional to the voltage difference between the voltage V_(S) and thevoltage produced from reference voltage source 22, and hence the pumpcurrent is proportional to the oxygen concentration within the exhaustgas. The pump current value is output as a value of voltage appearingbetween the terminals of current sensing resistor 23.

When the air/fuel ratio is within the rich region, the voltage V_(S)will be higher than the output voltage from reference voltage source 22,and hence the output voltage from differential amplifier 21 will beinverted from the positive to the negative level. In response to thisnegative level of output voltage, the pump current which flows betweenelectrodes 16a and 16b of the oxygen pump element 18 is reduced, and thedirection of current flow is reversed. Thus, since the direction of flowof the pump current is now from the electrode 16b to electrode 16a,oxygen will be ionized by electrode 16a, so that oxygen will betransferred as ions through oxygen pump element 18 to electrode 16b, tobe emitted as gaseous oxygen within the gas holding chamber 13. In thisway, oxygen is drawn into gas holding chamber 13. The supply of pumpcurrent is thereby controlled such as to maintain the oxygenconcentration within the gas holding chamber 13 at a constant value, bydrawing oxygen into or out of chamber 13, so that the pump current I_(P)will always be proportional to the oxygen concentration in the exhaustgas, both for operation in the lean region and in the rich region of theair/fuel ratio. The value of the feedback compensation coefficientK_(O2) referred to above is established in accordance with the pumpcurrent value I_(P), in a K_(O2) computation subroutine. This subroutinecan for example be similar to a program which is described in U.S. Pat.No. 4,566,419. Specifically, the oxygen concentration representing valueV_(O2), determined in accordance with I_(P), is compared with a targetair/ruel ratio V_(ref) (which is determined in accordance with theengine parameters), and if V_(O2) <V_(ref), the computation K_(O2) -Δ isexecuted, while if V_(O2) ≧V_(ref), the computation K_(O2) +Δ isexecuted.

An operating sequence for the oxygen concentration sensor abnormalitydetection method of the present invention will now be described,referring to the operating flow of CPU 47 shown in FIGS. 4a and 4b. Thisabnormality detection function is provided as an abnormality detectionsubroutine of of the fuel supply routine, and therefore is executed eachtime the fuel supply routine is executed.

In the operating sequence, CPU 47 first judges whether or not activationof the oxygen concentration sensor has been completed (step 61). Thisdecision can be based for example upon whether or not a predeterminedtime duration has elapsed since the supply of heater current to theheater element 20 was initiated. If activation of the oxygenconcentration sensor has been completed, then the voltage V_(S) betweenelectrodes 17a and 17b of sensor cell element 19 is read in, and adecision is made as to whether or not V_(S) is 0 [V] (step 62). If V_(S)≠0 [V], then a timer A (not shown in the drawings) in CPU 47 is resetand starts counting up from zero. If on the other hand V_(S) =0 [V],then a decision is made as to whether or not a count value T_(A) oftimer A is greater than a time interval t₀ (step 64). If T_(A) ≧t₀, thenthe pump current value I_(P) is read in, and a decision is made as towhether or not I_(P) is higher than an upper limit value I_(PLH) (step65). If I_(P) >I_(PLH), then an excessively high value of pump currentis flowing while a condition of V_(S) =0 (V) is continuously maintained.This is taken as indicating a short-circuit between electrodes 17a and17b of the sensor cell element 19. A "sensor cell element short-circuit"display command is therefore issued to drive circuit 46b (step 66). IfI_(P) ≦I_(PLH), then a decision is made as to whether or not the valueof the feedback compensation coefficient K_(O2), computed by the K_(O2)computation subroutine, is higher than an upper limit value K_(O2LH)(step 67). If K_(O2) >K_(O2LH), then this indicates a condition in whichV_(S) is continuously maintained at 0 (V) while an excessively highvalue of pump current flows in the positive direction, and the feedbackcompensation coefficient K_(O2) is excessively high. This condition istaken as indicating a short-circuit between electrodes 17a and 17b ofthe sensor cell element 19, and a "sensor cell element short-circuit"display command is therefore issued to drive circuit 46b (step 66).

After reset of timer A, if it is judged that the count value T_(A) oftimer A has not yet attained time t_(o), or if it is judged that K_(O2)≦K_(O2LH), then a decision is made as to whether or not voltage V_(S) isequal to Vcc (step 68). If V_(S) ≠Vcc (where Vcc is the circuit powersource voltage), then a timer B (not shown in the drawings) in CPU 47 isreset and counting up from zero by counter B is initiated (step 69). Ifon the other hand V_(S) =Vcc, then a decision is made as to whether ornot the count value T_(B) of timer B is greater than a time t₁ (step70). If T_(B) ≧t₁, then the value of pump current I_(P) is read in, anda decision is made as to whether or not I_(P) is smaller than a lowerlimit value I_(PLL) (step 71). If I_(P) <I_(PLL), then a condition ofV_(S) =Vcc is continuously maintained while an excessive level of pumpcurrent flow in the negative direction is occurring. This condition istaken as indicating an open-circuit in the connecting leads ofelectrodes 17a and 17b of the sensor cell element 19, and therefore a"sensor cell element open-circuit" display command is issued to drivecircuit 46b (step 72). If I_(P) ≧I_(PLL), then a decision is made as towhether or not the feedback compensation coefficient K_(O2) (computed inthe K_(O2) computation subroutine) is smaller than the lower limit valueK_(O2LL) (step 73). If K_(O2) <K_(O2LL), then this shows that althoughan excessively high level of pump current does not flow, a condition ofV_(S) =Vcc is continuing and the feedback compensation coefficientK_(O2) is excessively small. This is taken to indicate an open-circuitin the connecting leads of electrodes 17a, 17b of sensor cell element19, and a "sensor cell element open-circuit" display command is issuedto drive circuit 46b (step 72).

After timer B has been reset, if it is judged that the count value T_(B)does not yet correspond to the time value t₁, or if it is judged thatK_(O2) ≧K_(O2LL), then a decision is made as to whether or not the pumpcurrent value I_(P) is 0 (mA) (step 74). If I_(P) ≠0 (mA), then a timerC (not shown in the drawings) in CPU 47 is reset and begins counting upfrom zero (step 75). If on the other hand I_(P) =0 (mA), then a decisionis made as to whether or not the count value T_(C) of timer C is greaterthan the time t₂ (step 76). If T_(C) ≧t₂, then a decision is made as towhether or not the feedback compensation coefficient K_(O2) is greaterthan the upper limit value K_(O2LH) (step 77). If K_(O2) >K_(O2LH), thenthis is taken to indicate an open-circuit in the connecting leads ofelectrodes 16a, 16b of oxygen pump element 18 while the air/fuel ratiois more rich than the target air/fuel ratio, whereby a condition ofI_(P) =0 (mA) is being continued, so that the feedback compensationcoefficient K_(O2) is excessively high. An "oxygen pump elementopen-circuit" display command is therefore issued to drive circuit 46b(step 78). If K_(O2) ≦K_(O2LH), then a decision is made as to whether ornot the feedback compensation coefficient K_(O2) is smaller than thelower limit value K_(O2LL) (step 79). If K_(O2) <K_(O2LL), then this istaken to indicate that a condition of I_(P) =0 (mA) is continuouslyestablished while the feedback compensation coefficient K_(O2) isexcessively low due to an open-circuit in the connecting leads ofconnecting leads 16a, 16b of the oxygen pump element 18, so that theair/fuel ratio is more lean than the target air/fuel ratio. An "oxygenpump element open-circuit" display command is therefore issued to drivecircuit 46b (step 78).

After timer C has been reset, if it is judged that the count value T_(C)does not yet correspond to the time value t₂, or if it is judged thatK_(O2) ≧K_(O2LL), then a decision is made as to whether or not thefeedback compensation coefficient K_(O2) is greater than the upper limitvalue K_(O2LH) (step 80). If K_(O2) ≦K_(O2LH), then timers D and E(neither of which is shown in the drawings) in CPU 47 are respectivelyreset and begin counting up from zero (steps 81, 82). If on the otherhand K_(O2) >K_(O2LH), then a decision is made as to whether or not thecount value T_(D) of timer D is greater than a time interval t₃ (step83). If T_(D) <t₃, then timer E is reset and counting up by timer E fromzero is initiated (step 82). If T_(D) ≧t₃, then since it is unusual foran operating condition in which the air/fuel ratio is excessively leanto continue for more than time t₃, a decision is made as to whether ornot the voltage V_(S) between electrodes 17a and 17b of sensor cellelement 19 is lower than 0.4 (V) (step 84). If V_(S) <0.4 (V), then thisindicates that the air/fuel ratio is lean as a result of the value ofV_(S). Counter E is then reset and counting up by timer E from zero isinitiated (step 85). The engine speed of rotation N_(e) and the absolutepressure P_(BA) in the intake pipe are then read in, and a decision ismade as to whether or not the speed of rotation N_(e) is greater than3,000 (r.p.m.). In addition, a decision is made as to whether or not theabsolute pressure P_(BA) in the intake pipe is greater than 660 (mmH_(g)) (steps 86, 87), and a decision is made as to whether or not thetarget air/fuel ratio L_(ref) is set at a value lower than 14.7 (step88). The target air/fuel ratio may be determined in accordance withengine parameters representing engine load, such as N_(e) and P_(BA), byreading out a data map. If all of the predetermined conditions N_(e)≧3,000 (r.p.m.), P_(BA) ≧660 (mm H_(g)), and L_(ref) ≦14.7, aresatisfied, then this is taken to indicate that the engine is operatingunder a condition of high load with a rich air/fuel ratio. In this case,a decision is made as to whether or not the pump current I_(P) is higherthan the upper limit value I_(PLH) (step 89). If I_(P) >I_(PLH), thenthis shows that an excessively high level of pump current is flowing inthe positive direction, in spite of the fact that the air/fuel ratio isrich, and therefore this is taken to indicate that gas from the exhausthas leaked into the atmospheric reference chamber 15 due to a reasonsuch as a crack in the solid electrolytic member 11, so that the voltageV_(S) is lower than 0.4 (V). This is a "rich abnormality" detectioncondition, and a "rich abnormality detection" display command is issuedto drive circuit 46b (step 90). If at least one of the conditions N_(e)≧3,000 (r.p.m.), P_(BA) ≧660 (mm H_(g)), and L_(ref) ≦14.7 is notsatisfied, then a decision is made as to whether or not the pump currentI_(P) is greater than the lower limit I_(PLH) (step 91). If I_(P)>I_(PLH), then this indicates that although the feedback compensationcoefficient K_(O2) has been held continuously higher than the upperlimit value K_(O2LH), to compensate for a condition of lean air/fuelratio, and the air/fuel ratio has been thereby made rich, an excessivelyhigh level of pump current is flowing in the positive direction. This istaken to indicate that a short-circuit exists between electrodes 16a,16b of the oxygen pump element 18, and therefore an "oxygen pump elementshort-circuit" display command is issued to drive circuit 46b (step 92).The target air/fuel ratio L_(ref) is set in accordance with the enginespeed of rotation N_(e) and the absolute pressure P_(BA) in the intakepipe, in synchronism with the TDC signal, during a subroutine of thefuel supply routine.

If it is judged in step 84 that V_(S) ≧0.4 (V), then this value of V_(S)is taken as indicating that the air/fuel ratio is rich. Therefore, sinceit is unusual for the feedback compensation coefficient K_(O2) to exceedthe upper limit value K_(O2LH), a decision is made as to whether or notthe count value T_(E) of timer E has exceeded a time interval t₄ (step93). If T_(E) >t₄, then this is taken to indicate that there is anopen-circuit in the connecting leads of the heater element 20, or withinheater element 20 itself, and a "heater element open-circuit" displaycommand is issued to drive circuit 46b (step 94).

After resetting timer E in step 82 or if it is judged in step 93 thatthe count value T_(E) has not yet attained time t₄, a decision is madeas to whether or not the feedback compensation coefficient K_(O2) islower than the lower limit value K_(O2LL) (step 95). If K_(O2)≧K_(O2LL), then a timer F (not shown in the drawings) within CPU 47 isreset, and begins counting up from zero (step 96). If K_(O2) <K_(O2LL),then a decision is made as to whether or not the count value TF of timerF is greater than a time interval t₅ (step 97). If T_(F) ≧t₅, then adecision is made as to whether or not the voltage V_(S) betweenelectrodes 17a, 17b of sensor cell element 19 is greater than 0.4 (V)(step 98). If V_(S) ≦0.4 (V), then since such a value of V_(S) indicatesthat the air/fuel ratio is lean. Since it is unusual for the feedbackcompensation coefficient K_(O2) to fall below the lower limit valueK_(O2LL), this condition is taken to indicate that there is anopen-circuit in the connecting leads of heater element 20 or withinheater element 20 itself, and a "heater element open-circuit" displaycommand is issued to drive circuit 46b (step 94). If on the other handV_(S) >0.4 (V), a decision is made as to whether or not the pump currentI_(P) is smaller than the lower limit value I_(PLL) (step 99). If I_(P)<I_(PLL), then this indicates that in spite of the fact that thefeedback compensation coefficient K_(O2) has been continuously heldlower than the lower limit value K_(O2LL) to compensate for a conditionof rich air/fuel ratio, so that the air/fuel ratio has been made lean,an excessively high level of pump current is flowing in the negativedirection. This is taken to indicate that there is a short-circuitbetween electrodes 16a and 16b of the oxygen pump element 18, andtherefore an "oxygen pump element short-circuit" display command isissued to drive circuit 46b (step 92).

If a "sensor cell element short-circuit" display command, a "sensor cellelement open-circuit" display command, or a "heater elementopen-circuit" display command has been issued, then the feedbackcompensation coefficient K_(O2) is made equal to 1, in order to haltair/fuel ratio feedback control operation (step 100). If an "oxygen pumpelement short-circuit" display command or an "oxygen pump elementopen-circuit" display command has been issued, then, operating under theassumption that the sensor cell element 19 is functioning normally,processing is executed to compute the feedback compensation coefficienton the basis of judgement of the air/fuel ratio in accordance with thevalue of voltage V_(S) developed between electrodes 17a and 17b ofsensor cell element 19 (step 101).

When drive circuit 46b is supplied with any of the display commandsdescribed above, a predetermined display pattern is produced upon adisplay unit 38, in accordance with the contents of the command.

Each of the timers A through F can be implemented by supplying clockpulses to vary the contents of registers within CPU 47.

In the embodiment of the present invention described above, the fuelsupply quantity is controlled in accordance with a pump current valueI_(P). However, the present invention is not limited to such anarrangement. It would be equally applicable to a method of abnormalitydetection for an oxygen concentration sensor of an air/fuel ratiocontrol apparatus which employs a secondary air intake technique,whereby a quantity of secondary intake air could be controlled inaccordance with the pump current I_(P).

Furthermore, in the embodiment of the present invention described above,the lead-in aperture 14 is utilized as a gas diffusion control means.However, it is also possible to employ a porous material such as alumina(Al₂ O₃), formed as a porous body which fills a lead-in aperture or thegas holding chamber.

With a system of abnormality detection for an oxygen concentrationsensor according to the present invention as described hereinabove, ifan abnormality such as an open-circuit or a short-circuit in electrodeconnecting leads of a sensor cell element should occur, then the voltagewhich is developed between the electrodes of the sensor cell elementwill become constant, while at the same time an excessive level ofcurrent will flow between the electrodes of the oxygen pump element. Asa result, an air/fuel ratio compensation value, such as K_(O2) describedabove, derived in accordance with an oxygen concentration detectionvalue for use in controlling the air/fuel ratio of a mixture supplied toan engine such as to hold that air/fuel ratio at a target value, willbecome excessively high or excessively low. Thus, an abnormality in theelectrode connection system of the sensor cell element can be reliablydetected by the method of the present invention on the basis of thevoltage developed between the electrodes of the sensor cell element inconjunction with the current which flows between the electrodes of theoxygen pump element, or on the basis of the voltage developed betweenthe electrodes of the sensor cell element, in conjunction with theair/fuel ratio compensation value. With an abnormality detection methodaccording to the present invention, in the event that an abnormality isdetected, then air/fuel ratio control in accordance with data from theoxygen concentration sensor is immediately halted. This serves toprevent the engine from being operated under a condition of loweredaccuracy of control of the air/fuel ratio of the mixture which issupplied to the engine. In this way, the invention enables deteriorationof pollution cleansing effectiveness to be effectively prevented.

What is claimed is:
 1. A system for abnormality detection in an oxygenconcentration detector comprising:an oxygen concentration sensor whichincludes two pairs of electrodes disposed in mutual opposition with eachof the pairs sandwiching an oxygen ion-conductive solid electrolyticmember; gas diffusion control means for providing a gas which is undermeasurement to the vicinity of one electrode of each of said two pairsof electrodes; control means for applying a pump voltage, determined inaccordance with a voltage difference between a sensor voltage which isproduced between a first pair of said two pairs of electrodes and areference voltage determined between a second pair of said two pairs ofelectrodes to maintain said sensor voltage at said reference voltage,said control means thereby producing, as an output which represents anoxygen concentration sensing value, a value of pump current which flowsbetween said second pair of electrodes; and means for detecting anabnormality of an electrical connection system of said first pair ofelectrodes based upon a voltage developed between said first pair ofelectrodes and upon a current which flows between said second pair ofelectrodes.
 2. An abnormality detection system according to claim 1,wherein; when a condition occurs in which a voltage developed betweensaid first pair electrodes is below a first predetermined value while acurrent which flows between said second pair of electrodes is higherthan a second predetermined value, then said condition is judged asindicating an abnormality in said system of electrical connections ofsaid first pair of electrodes
 3. An abnormality detection systemaccording to claim 1, wherein, when a condition occurs in which avoltage developed between said first pair of electrodes is higher than athird predetermined value while a current which flows between saidsecond pair of electrodes is lower than a fourth predetermined value,said condition is judged to indicate an abnormality in said system ofelectrical connections of said first pair of electrodes.
 4. A system forabnormality detection in an air/fuel ratio control apparatus for aninternal combustion engine, said engine comprising:an oxygenconcentration sensor including two pairs of electrodes disposed mutuallyopposing with each of the pairs sandwiching an oxygen ion-conductivesolid electrolytic member; gas diffusion control means, communicatingwith an exhaust gas of said internal combustion engine, for providingsaid gas to the vicinity of one electrode of each of said two pairs ofelectrodes; control means for applying a pump voltage, determined inaccordance with a voltage difference between a sensor voltage which isproduced between a first pair of said two pairs of electrodes andreference voltage determined between a second pair of said two pairs ofelectrodes to maintain said sensor voltage at said reference voltage,said control means thereby producing, as an output which represents anoxygen concentration sensing value, a value of pump current which flowsbetween said second pair of electrodes; means for obtaining an air/fuelratio compensation value based upon said oxygen concentration sensingvalue, for use in controlling an air/fuel ratio of a mixture supplied tosaid internal combustion engine to maintain said ratio at a targetair/fuel ratio; and drive means for driving air/fuel ratio adjustmentmeans of said internal combustion engine in accordance with a correctedair/fuel ratio control value obtained by correcting said air/fuel ratiocontrol value by said air/fuel ratio compensation value; and means fordetecting an abnormality of an electrical connection system of saidfirst pair of electrodes based upon a sensor voltage developed betweensaid first pair of electrodes and upon said air/fuel ratio compensationvalue.
 5. An abnormality detection system according to claim 4, whereinwhen a condition occurs in which a voltage developed between said firstpair of electrodes is below a first predetermined value while a currentwhich flows between said second pair of electrodes is is higher than asecond predetermined value, said condition is judged to indicate anabnormality in said system of electrical connections of said first pairof electrode.
 6. An abnormality detection system according to claim 4,wherein when a condition occurs in which a sensor voltage developedbetween said first pair of electrodes is higher than a thirdpredetermined value and a current which flows between said second pairof electrodes is lower than a fourth predetermined value, said conditionis judged to indicate an abnormality in said system of electricalconnections of said first pair of electrode.
 7. An abnormality detectionsystem according to claim 4, wherein when a condition occurs in which avoltage developed between said first pair of electrodes is lower than afirst predetermined value and said compensation value is higher than afifth predetermined value, said condition is judged to indicate anabnormality in said system of electrical connections of said first pairof electrode.
 8. An abnormality detection system according to claim 4,wherein when a condition occurs in which a voltage developed betweensaid first pair of electrodes is higher than a third predetermined valueand said compensation value is lower than a sixth predetermined value,said condition is judged to indicate an abnormality in said system ofelectrical connections of said first pair of electrode.